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The Effect of Deflection on Floor Flatness

ASCC Position Statement #36

ACI 117-06 and ACI 117-10, “Specifications for Tolerances for Concrete Construction and Materials,” require F-numbers to be measured within 72 hours after slab concrete placement. This was not always the case, as ACI 117-90 provided no time requirement for the measurement of floor flatness, FF, and the commentary stated the reason: “Since neither deflection nor curling will significantly change a floor’s FF value, there is no time limit on the measurement of this characteristic.”

The statement that deflection will not significantly change a floor’s FF value has since been shown to be incorrect. This is why ACI 117-06 and ACI 117-10
require that FF measurements be made within 72 hours after completion of slab concrete finishing operations and before removal of any supporting shores. ACI requires the measurements to exclude the effect of deflection on the floor flatness.

ACI 435R-95, “Control of Deflection in Concrete Structures,” states that deflection “can only be estimated in the range of 20-40 percent accuracy.” ACI 318-08 indicates that shrinkage and creep cause additional long-term deflection of 1.0 to 1.4 times the initial deflection for times between 3 to 12 months. Thus, when interior finishes are applied at this age, the total floor deflection might be 2 to 2.4 times the initial deflection. ACI 318-08 also requires that “drawing, details, and specifications shall show: provision for dimensional changes resulting from creep, shrinkage, and temperature.”

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